But it’s a Rock…

Even though Alcatraz is a rock, greywackle sandstone to be exact, erosion is taking its toll.

The Warden’s house in 1924. Photo courtesy of the San Francisco Library

Originally, the island had two summits that were blasted down by the military when the island was used as a fortress and later a military prison. Steep cliffs were created and the roadways that we walk today were carved into the hillsides. The Commander of the Army, and later, the Warden of the Federal Prison, picked the best vantage point to build his home on Alcatraz, of course, he picked the top of the island overlooking the Parade Ground.

The Warden’s house and garden in the 1940s. Photo courtesy of Chuck Stucker

The cliff that the Warden’s House is perched upon has been slowly crumbling over the past few decades. This month, work was started to stabilize the cliff with placing shotcrete, to preserve the cliff and the historic home. Shotcrete is a mortar that is more dense, homogenous, stronger and waterproof compared to concrete. Shotcrete is sprayed onto the cliff, usually over a mesh, through a hose at high speed, so it can even be applied to vertical surfaces – making it a great choice to stabilize cliffs.

The cliff below the Warden’s house in 2008. Photo by Owen Shui

Workers first dangled over the cliff to remove vegetation and loose rock. A huge pile of Agave americana soon accumulated. These plants are truly amazing to cling to rock.

This week, the crew color coded the locations for placement of dowels that would be used to secure the mesh and bond the shotcrete (white) to the cliff face, weep holes to provide drainage (blue) and the locations of deeper rock bolts (orange).

The cliff being stabilized. Photo by Shelagh Fritz

The work to stabilize the cliff will continue to the end of January. Visitors can watch the work progress and it is pretty interesting to see preservation of this historic landmark in progress.